Aircraft have used on-board inert gas generating systems (OBIGGS) to protect against fuel tank explosions by replacing the potentially explosive fuel vapor/air mixture above the fuel in the ullage space of the tanks with nitrogen-enriched air (NEA). The NEA is generated by separating oxygen from local, ambient air and pumping the inert, oxygen depleted NEA into the tanks.
Production of NEA typically is carried out by means of an apparatus relying on permeable membranes, or on molecular sieves. The air separation apparatus is generally referred to as an air separation module (ASM). In systems utilizing permeable membranes, the ASM typically comprises a bundle of hollow fiber membranes packaged in a cylindrical shell with an inlet and outlet at the ends of the shell, and a shell side vent port. When pressurized air enters the ASM inlet port and passes through the hollow fibers, oxygen is separated from the air stream due to diffusion through the fiber walls. That is, the fiber walls are more permeable to oxygen than nitrogen. Oxygen enriched air (OEA) exits through the side vent port and can be recaptured, but often the OEA is considered a waste gas that is exhausted overboard. The remaining NEA flows out of the ASM via the outlet port and is distributed to the ullage space of the fuel tank or tanks for the purpose of inerting the fuel tanks and thereby their reducing flammability. The ASM operates most efficiently, in terms of permeability of oxygen over nitrogen, at an elevated temperature higher than ambient temperature. The selective permeability has a direct relationship to the purity of the NEA (the more nitrogen and less oxygen, the higher the purity).
In many if not most commercial airplane applications, pressurized air used for NEA generation will originate from either an engine bleed or from a cabin air pressure source. With an engine bleed pressure supply, compressed hot air is ducted from an engine bleed air supply line and then cooled by a heat exchanger to an optimal temperature for maximum ASM performance and life.
The flow rate of NEA to a fuel tank generally depends on the stage of the aircraft's flight. On ascent, a low flow of NEA may typically be used because the fuel tank is full and the fuel tank is being depressurized. On descent, the flow rate of NEA to a fuel tank is typically higher, as there is less fuel in the fuel tank and the fuel tank is being re-pressurized.
It is conventional practice to include filtration upstream of the ASM to remove particulate and aerosols that may exist in the bleed air, since they can potentially foul, plug, or otherwise degrade the ASM if ingested.
In addition, bleed air can contain gaseous vapors originating from various organic-based fluids that are used in and around the aircraft, e.g., jet fuel, hydraulic fluid, engine turbine oil, de-icing fluid, cleaning agents, etc., collectively known as VOCs (Volatile Organic Compounds). It is known that VOC exposure is detrimental to ASMs, as the VOCs will foul ASM fibers and reduce their performance, and may even significantly impact ASM durability. For these reasons, industrial, ground-based air separation systems utilizing ASM technology commonly employ filtration to remove VOCs upstream of the ASM inlet. Typically, one or more active carbon towers are used for this purpose, the size of which can be substantial in comparison to the ASMs which the active carbon towers are protecting.
In contrast, current aircraft inerting systems make no attempt to filter or remove vapor species from the airstream, despite the knowledge that VOC exposure can be very detrimental to ASM performance and life. The lack of a vapor contaminant removal system is primarily due to the significant size and weight penalties that are believed to be inherent to this system, which would be intolerable in the highly weight-sensitive aircraft industry.